1. Field of the Invention
The present invention relates to a method of detecting an element resistance of a gas concentration sensor that detects the concentration of a gas in the exhaust of a vehicle internal combustion engine via a voltage-current frequency characteristic.
2. Description of Related Art
There have been demands in recent years for improved control accuracy in the air-fuel ratio control of motor vehicle engines to maintain engines in a lean-burn state. In response to these demands, a linear air-fuel ratio sensor, or oxygen-concentration sensor, for detecting the air-fuel ratio of mixed air linearly supplied to an internal combustion engine over a wide zone, has been developed and utilized. With such an air-fuel ratio sensor, it is important to keep the air-fuel ratio sensor in an active state to ma the sensor's detection accuracy. This is typically accomplished by electrically controlling a heater attached to the air-fuel ratio sensor to heat a sensor element of the air-fuel ratio sensor to thereby maintain the active state.
To electrically control the heater, conventional techniques detect the temperature of the sensor element and execute a feedback-based control method so that the element temperature is a desired active temperature (for example, about 700.degree. C.). A temperature sensor is attached to the sensor element, and the element temperature is periodically derived from the detection result of the temperature sensor. However, the temperature sensor adds significant cost to the air-fuel ratio sensor. For example, in Japanese Patent Laid-Open No. Sho 59-163556, the sensor element resistance is measured, as the resistance of the sensor element has a predetermined corresponding relation with the element temperature. The element temperature is derived from the detected element resistance.
The time of the element resistance detection has a predetermined cycle. When the cycle continuously coincides with a cycle of the air-fuel ratio detection of a specific cylinder of a multi-cylinder internal combustion engine having at a practical engine rotational speed, a fluctuation in output of the air-fuel ratio in a transition state at a time of combustion in the specific cylinder cannot be detected.